1. Field of the Invention
The present invention relates to an optical recording medium and an optical information apparatus in which information is optically reproduced, recorded or erased by using a light beam or the like, and a tracking method, and particularly to a method and a device for tracking correction.
2. Description of the Related Art
Recently, techniques of reproducing or recording information in a high density by using a light beam are well known, and in practical use for an optical disk. In order to accurately reproduce data from or record data into tracks of such an optical disk, deflections of the light beam spot with respect to tracks which may be caused by eccentricity of the center hole or tracks of the optical disk, runout of the rotation shaft of the turn table, etc. must be corrected so that the light beam spot accurately follows the tracks (or conducts a tracking).
A typical tracking method is the push-pull method which uses guide grooves preformatted on an optical disk. In the push-pull method, track deviation is detected by using a diffraction distribution of a spot of a light beam which is impinged on a guide groove. It is known that, in the push-pull method, an offset due to the tilt of a disk or a movement of a light beam spot on an optical detector for detecting track deviation is produced so that there arises an error between the actual deviation of the light beam spot with respect to the track and detected track deviation.
As a method which can solve this problem, known is the wobble pit method in which track deviation is detected by means of a pair of wobble pits formed laterally on both sides of the track center. In the wobble pit method, the above-mentioned offset is produced at a low level and a tracking error signal can satisfactorily be obtained. However, a large number of wobble pits must be preformatted on an optical disk, thereby producing a problem in that the area for the preformatting reduces the disk recording capacity.
In order to comply with this, proposed is an optical information recording and reproduction apparatus using a composite tracking method in which the above-mentioned push-pull method and wobble pit method are combined with each other (Japanese Laid-open Patent Publication No. 62-89241). According to the composite tracking method, wobble pit signals are detected from light reflected from an optical disk's surface, a peak level difference between a pair of wobble pit signals is calculated, and the above-mentioned offset is reduced by subtracting the difference from a tracking error signal obtained by the push-pull method, so that a light beam spot always accurately follows the center of the track.
Another configuration is proposed in which land and groove tracks are alternately arranged in the radial direction of an optical disk and information is recorded in both the land and groove tracks so that a high density recording is realized (Japanese Laid-open Patent Publication No. 57-50330).
In the conventional composite tracking method, as described above, the accuracy of the detection of wobble pits largely affects the tracking accuracy. The above documentation of the above conventional optical information recording and reproduction apparatuses fail to describe a specific configuration and method of detecting wobble pits. The conventional apparatus conducts a sampling process on the premise that an optical disk is rotated in an ideally constant manner at a constant speed. Actually, however, it is very difficult to accurately detect wobble pits because of various reasons such as rotation unevenness of the motor for rotating the optical disk, and eccentricity of the optical disk.
In the following, the case where an optical disk which is divided into a plurality of zones is subjected to a continuous recording/reproduction process will be considered. When a continuous recording/reproduction process is conducted across zones by the MCLV method, it is impossible to, in each of the zones, instantaneously change the rotation speed of the motor for rotating the optical disk to the speed corresponding to the zone. Therefore, a light beam spot is moved to the next zone while maintaining a substantially constant rotation speed. Also in the MCAV method, the rotation speed of the motor for rotating the optical disk is kept constant, and therefore a light beam spot is moved to the next zone while maintaining a substantially constant rotation speed in the same manner as the MCLV method. In a continuous recording/reproduction process conducted across zones, consequently, the difference in pit density between the zones causes the frequency of a reproduced pit signal to be suddenly changed. Also in an optical disk divided into a plurality of zones, accordingly, it is very difficult to accurately detect wobble pits.
In actuality, it is impossible to completely eliminate a lack of wobble pits or a formation of pseudo pits due to defects or the like. There arises a problem that erroneous detection of wobble pits which is due to such an optical disk largely impairs the accuracy of the tracking correction.
In an optical disk wherein information is recorded in both land and groove tracks, in order to realize a high density recording, the polarity of a push-pull tracking error signal which is produced by using a guide groove preformatted on the optical disk is inverted at each of the land and groove tracks. This produces a problem that information recorded in both the land and groove tracks cannot be reproduced unless the tracking correction direction is inverted at each of the land and groove tracks.